In a typical CCD imager, signal charge representative of incident radiation is accumulated in an array of pixels in an image area. Following an integration period, signal charge is transferred to a store section and then to an output register by applying appropriate clocking or drive pulses to control electrodes. The signal charge is then read out from the output register and applied to a charge detection circuit to produce a voltage that is representative of the amount of signal charge. The sensitivity of such a device is limited by the noise of the charge to voltage conversion process and that introduced by the subsequent video chain electronics.
An electron multiplying CCD (EMCCD) overcomes this limitation and is disclosed in our earlier published UK patent application GB-A-2,371,403, as shown in FIG. 1. A CCD imager 1 comprises an image area 2, a store section 3 and an output or read-out register 4, each of these components being found in a conventional CCD imager. The output register 4 is extended serially to give a multiplication register 5, the output of which is connected to a charge detection circuit 6.
During operation of the device, incident radiation is converted at the image area 2 into signal charge which is representative of the intensity of the radiation impinging on the array of pixels making up the image array. Following the image acquisition period, drive pulses are applied to control electrodes 7 to transfer the charge accumulated at the pixels of the image area 2 to the store section 3. Simultaneously with this, drive signals are also applied to control electrodes 8 at the store section 3 to cause charge to be transferred from row to row as indicated by the arrow, the last row of charge held in elements in row 3 being transferred in parallel to the output register 4.
When a row of signal charge has been transferred into the output register 4, appropriate drive pulses are applied to the electrodes 9 to sequentially transfer the charge from the elements of the output register to those of the multiplication register 5. In this embodiment, the multiplication register is of similar architecture to the output register in so far as doping is concerned with the addition of an electrode for multiplication.
To achieve multiplication of charge in each of the elements of the multiplication register 5, sufficiently high amplitude drive pulses are applied to control electrodes 10 to both transfer signal charge from one element to the next adjacent element in the direction shown by the arrow and also to increase the level of signal charge due to impact ionisation by an amount determined by the amplitude of the drive pulses. Thus, as each packet of charge is transferred from one element to the next through the multiplication register, the signal charge increases. The charge detected at circuit 6 is thus a multiplied version of the signal charge collected in the output register 4. At each stage of the multiplication register, the signal charge is increased. Each signal charge packet stored in the output register 4 undergoes an identical multiplication process as each travels through all the elements of the multiplication register 5.
The output of the charge detection circuit 6 is also applied to an automatic gain control circuit 11 that adjusts the voltages applied to the multiplication register 5 to control the gain. In other embodiments, this feedback arrangement is omitted. Gain may then be controlled manually if desired.
Whilst the gain control circuit can vary the gain provided by varying the voltages applied to the multiplication register, we have appreciated the need to determine the actual level of gain provided by such a CCD charge multiplication arrangement. One way to measure the gain is to inject a known amount of signal into the multiplication register and monitor the output. The difficulty with this approach is knowing what the input signal is. Typically this signal will be below the noise floor of the video chain if multiplication gain is not applied. Therefore measuring the signal with and without gain is not a practical proposition.
The approach usually taken to measure the gain is to illuminate the device and to measure the output with no multiplication gain. The light level is then reduced by a known fraction (by reducing the aperture of the optics or using neutral density filters for example). The reduction of light level will be of the same order as the gain to be measured. Multiplication gain is then applied and the output signal is measured. The multiplication gain can be calculated knowing the output signal and the reduction in light level. This method can give accurate results but is cumbersome and not particularly suitable for automatic measurements within a camera system.
It has been proposed that the distribution of output signal can be used to calculate the multiplication gain. This method involves the analysis of the statistical variation of the output signal from very small input signals. The input signals derive from sources such as dark signal but signals derived from the scene being viewed are not used. The main disadvantage of this method is that light from the scene must be prevented from reaching the sensor during the measurement of gain. This would require an efficient shutter arrangement or the application of some other optical shielding. In many cases this is not practically possible or desirable.
We have appreciated that an improved method of determining the gain of an electron multiplying CCD would be desirable. We have further appreciated that a method that does not require illumination with known relative light levels, or shuttering of the CCD, would be simpler to implement and operate during normal use of a CCD.